The invention is related to polygraphy, and, in particular, to the production of polymer layers with latent images visible in polarized light that can be used as protective marks on various documents, security papers, banknotes as well as for manufacturing of excise documentary stamps, labels, tags and other products of the kind.
At present to prevent forgery of various kinds of products the latter are supplied with some peculiar features that are difficult to reproduce such as watermarks, micro-range printing, embedded metal strips. As a kind of such protection there can also be used optical elements that are capable of varying the polarization of incident light such as holograms, liquid-crystal optical elements as well as polymer layers with latent image visible exclusively in polarized light.
The latter are produced as a rule by varying the anisotropic properties of the separate areas of a polymer layer thus forming a latent image.
The above-described modification can also be provided by selectively varying the thickness of a polymer film by mechanic [U.S. Pat. No. 5,284,364 A, Feb. 8, 1994] or thermal mechanic [U.S. Pat. No. 4,659,112 A, Apr. 21, 1987] means or with the help of laser radiation [GB 2328180 A, Feb. 17, 1999].
Also known are the methods of producing a latent image by means of selective photo-stimulation of a light-sensitive polymer layer [RU 2165360 C1, Feb. 24, 1999, U.S. Pat. No. 6,124,970 A, Sep. 26, 2000, U.S. Pat. No. 5,389,698 A, Feb. 14, 1995].
For example, it is known a method of producing a latent image comprising the steps of treatment of originally light-sensitive anisotropic polymer by the solution containing a photo-activating substance, selective irradiation to form the areas with different anisotropic characteristics as compared to the original ones and then fixing the latent image thus received [U.S. Pat. No. 6,124,970 A, Sep. 26, 2000].
The most closely related to the method filed is a method of producing a polymer layer with a latent polarized image comprising the steps of preparing a 2% polymer solution in an organic solvent, application of said solution on a light-reflecting substrate, further drying to produce an optically isotropic polymer layer and generating there on the said layer of the areas with anisotropic properties by means of irradiation through a mask by a Hg lamp [U.S. Pat. No. 5,389,698 A, Feb. 14, 1995].
However, all of the above-described methods do not provide one of the most important requirements to a polymer layer thus received which enable its further use as a protective mark or a constituent part thereof i.e. no contours or traces of a polarized image being evident when visualized in the usual way. Besides, the products produced by the above-described methods do not have the required stability with regard to UV 20 radiation and high temperatures and have limited field of application.